RFID tags are becoming a well-established method for tracking materials during shipping and storage. In many applications they replace the printed bar code labels on items because they do not require a close proximity for the automatic reader. RFID tags that conform to the ISO/IEC 18000 or epcGlobal standards also can contain significantly more data than a printed bar code label and can be modified en route to include waypoint or other information.
There are two main classes of RFID tags: passive and active. A passive tag derives its energy for operation from the RF field of the reader and responds to the reader by modulating the reflectivity of its antenna, thereby returning varying amounts of the incident RF energy. An active tag responds to the reader using RF energy derived from a battery on the tag.
Present RFID tags are fabricated by electrically bonding a single custom integrated circuit (IC) microradio to a substrate containing a printed circuit antenna. The usual fabrication method, well known in the electronics industry is flip-chip bonding using pick-and-place machines.
However, the use of a single integrated circuit microradio chip presents certain problems. First is the operability of the chip itself. Production yields for the IC are not high enough to guarantee operability without testing. Testing individual ICs either prior to tag mounting or after tag manufacture is a very costly and complicated procedure, especially when tags are miniaturized for item-level tagging. Also pick-and-place techniques cannot guarantee proper chip coupling to the antenna due to the small sizes involved. Moreover, ever smaller sizes are desirable to reduce the cost of the chips and thus the cost of the tag to under 5 cents, necessitating other chip mounting techniques.
One such chip mounting technique is described in PCT Application Serial No. US2006/033,111 filed Aug. 24, 2006 by Kenneth R. Erikson, entitled “RFID Tag and Method and Apparatus for Manufacturing Same,” which describes a method for fabricating a low cost RFID tag wherein at least two integrated circuits, and preferably hundreds, are deposited at the feed point of the tag antenna. In one embodiment the integrated circuit microradio chips are mixed in a non-conductive slurry that is printed onto the antenna feed point, followed by patterning electrodes and programming.
PCT Application Serial No. US2006/033,112 filed Aug. 24, 2006 by Karl D. Brommer and Kenneth R. Erikson, entitled “Coherent Multichip RFID Tag and Method and Apparatus for Creating such Coherence,” describes a method for causing the multiple integrated circuit microradio chips to work together cooperatively.
Causing these microradio chips to work cooperatively also can include polarization control described in PCT Patent Application No. US2006/033,112, filed Aug. 24, 2005 by Steve A. Hedges et al.